1. Field of the Invention
The present invention relates to a controlled static convertor having terminals for connection to an alternating voltage source and comprising controlled rectifiers and a control pulse device arranged to deliver control signals to the rectifiers, the control pulse device comprising means for affecting the control angle of the rectifiers and means for limiting the control angle to an upper limit value.
2. The Prior Art
A single-phase controlled static convertor is known which has four thyristors which receive ignition pulses from a control pulse device. The AC terminals of the convertor are connected to an alternating voltage source with a voltage U.sub.AC. The DC side of the convertor is connected to a load object, such as a DC motor. The direct voltage of the convertor is controlled by phase-angle control, by varying the phase position (in relation to the alternating supply voltage) of the ignition pulses to the thyristors. The thyristors can be ignited in principle at any time during each half-cycle.
If the ignition pulse is delivered at the beginning of the half-cycle (control angle .beta. = 0), the highest possible value of the direct voltage will be obtained. If the ignition pulse is delivered at the end of the half-cycle (control angle .alpha. = 180.degree.), the lowest value of the direct voltage will be obtained. If the inductance in the DC circuit and the average value of the direct current are of sufficient magnitude, the direct current will at each moment be greater than zero. The convertor will then work with continuous direct current.
In order to obtain a safe commutation under all circumstances during inverter operation (.alpha. &gt; 90.degree.) when the direct current is continuous, the control angle must be limited, in a known manner, to a value which is less than 180.degree., for example 150.degree. (.alpha.=.alpha..sub.max). This can be achieved by limiting the control signal, or by making certain that an ignition pulse is always delivered when .alpha.= .alpha..sub.max, regardless of whether an ignition pulse has previously been delivered or not. By taking this measure, also, the absolute value of the attainable negative direct voltage is limited to a value which is less than U.sub.D, (the highest possible value of a direct voltage U.sub.D, which is reached when the control angle .alpha. is 0, which, however, does not cause any serious disadvantage in this mode of operation.
In the case of a purely resistive load the direct current will be discontinuous, that is it declines to zero for at least a short moment during each half-cycle.
In practice, the load object as a rule contains both resistive and inductive components and also a counter-electromotive force. In this case, when the direct current (its average value) is above a certain value, the current will be continuous. When the average current is below that value, the current will be discontinuous.
In order to secure the commutation in case of continuous direct current, it is necessary to limit the control angle, as mentioned above. This means that, with discontinuous direct current, it will not be possible to decrease the direct voltage U.sub.D to zero. This causes several serious drawbacks, for example the following in the case of motor drives:
a. If the motor is supplied with current from a single convertor, its speed cannot be reduced to zero at low load. PA0 b. If the motor is supplied with current from a double convertor, unwanted pole reversals at a high frequency are obtained at zero speed, which may cause great stresses on the motor and the load. PA0 c. In some cases, for example in case of inverter operation with a high counter-electromotive force, the direct current may become uncontrollable.
The disadvantages can be reduced, but not completely remedied, by increasing the inductance in the DC circuit, for example with the help of a reactor connected in series with the load object. However, such a reactor is heavy, space-demanding and expensive, and it reduces the control speed.
According to the invention, therefore, there is provided in this type of controlled static convertor a control pulse device which includes means for affecting the control angle of the rectifiers and means for limiting the control angle to an upper limit value. Means are provided responsive to a discontinuous direct current to render the control angle limiting means inactive, and also means are provided for sensing the direct current of the convertor and, if the direct current at a time corresponding to the upper limiting value of the control angle is less than a predetermined limiting value, to render the control angle limiting means inactive.